Virus-like particles (VLPs) are particles derived in part from viruses through the expression of certain viral structural proteins which make up the viral envelope and/or capsid, but VLPs do not contain the viral genome and are non-infectious. VLPs have been derived for example from the Hepatitis B virus and certain other viruses, and have been used to study viral assembly and in vaccine development.
Viral capsids are composed of at least one protein, several copies of which assemble to form the capsid. In some viruses, the viral capsid is covered by the viral envelope. Such viral envelopes are comprised of viral glycoproteins and portions of the infected host's cell membranes, and shield the viral capsids from large molecules that would otherwise interact with them. The capsid is typically said to encapsidate the nucleic acids which encode the viral genome and sometimes also proteins necessary for the virus' persistence in the natural environment. For the viral genome of a virus to enter a new host, the capsid must be disassembled. Such disassembly happens under conditions normally used by the host to degrade its own as well as foreign components, and most often involves proteolysis. Viruses take advantage of normal host processes such as proteolytic degradation to enable that critical part of their cycle, i.e. capsid disassembly and genome release.
It is therefore unsurprising that the literature has not previously described capsids resistant to hydrolases that act on peptide bonds. A very limited number of certain specific peptide sequences which are part of larger proteins are known to be somewhat resistant to certain proteases, but the vast majority of peptide sequences are not. Viruses that resist proteolysis have been reported, but these are all enveloped viruses, in which the capsid is shielded by the viral envelope. In such viruses the capsids are not in contact with, i.e. they are shielded from, the proteases described. Thus the role, if any, of the proteolytical stability of the virus capsid in such cases is unknown.
In large-scale manufacturing of recombinant molecules such as proteins, ultrafiltration is often used to remove molecules smaller than the target protein in the purification steps leading to its isolation. Purification methods also often involve precipitation, solvent extraction, and crystallization techniques. These separation techniques are inherently simple and low cost because, in contrast to chromatography, they are not based on surface but on bulk interactions. However, these techniques are typically limited to applications to simple systems, and by the need to specify a different set of conditions for each protein and expression system. Yet each target recombinant protein presents a unique set of binding interactions, thereby making its isolation process unique and complex. The separation efficiency for recombinant proteins using these simple isolation processes is therefore low.
Nucleic acids, including siRNA and miRNA, have for the most part been manufactured using chemical synthesis methods. These methods are generally complex and high cost because of the large number of steps needed and the complexity of the reactions which predispose to technical difficulties, and the cost of the manufacturing systems. In addition, the synthetic reagents involved are costly and so economy of scale is not easily obtained by simply increasing batch size.
Chemically and enzymatically synthesized RNA is commonly used for RNAi applications, mostly for down-regulation or repression of expression of proteins. Examples of RNA delivered into organisms for up-regulation of expression of endogenous proteins or expression of exogenous proteins are very limited. A previously described method for delivering mRNA to the body is limited to 5′ capped RNA which is difficult to synthesize. A need remains for improved and cost-effective RNA delivery methods and methods for both RNAi and mRNA applications.